Former News of the World editor Rebekah Brooks has spoken of her "shock and horror" when she learned that the newspaper hacked murdered schoolgirl Milly Dowler's voicemails while she was editor.

The 45-year-old told the Old Bailey she only found out on July 4, 2011 that 13-year-old Milly's messages had been illegally accessed by the newspaper, and had known nothing about the tasking of phone hacker Glenn Mulcaire at the time.

As she returned to the witness box for the third day, the former News International chief executive was questioned about the 2002 story of the missing Surrey schoolgirl, which was to go on to bring about the Sunday tabloid's downfall.

Brooks, who denies conspiring to hack phones, conspiring to commit misconduct in public office and conspiring to cover up evidence to pervert the course of justice, revealed that she did not know when she was editing the NotW that hacking was illegal.

She also told the court that hypothetically, if there was a strong enough public interest for a story, she might have authorised hacking while she was editor.

Asked about her reaction when she found out in 2011 that Milly's phone had been hacked by the NotW when she was at the helm, Brooks, of Churchill, Oxfordshire, said: "Shock, horror, everything.

"I was told that the NotW had asked someone to access Milly Dowler's phone while she was missing, that they had also deleted her voicemails and for a period of time because of that her parents had been given false hope and thought she was alive.

"I just think anyone would think that that was pretty abhorrent, so my reaction was that. That was what I was told."

Milly disappeared on March 21, 2002, the court heard, and while still missing, her voicemails were hacked the following month, between April 10 and 12.

The court heard that Brooks was on holiday in Dubai with then-partner Ross Kemp from April 7 - 14, leaving her then-deputy Andy Coulson, a co-defendant in the trial, in charge.

Under questioning from her lawyer Jonathan Laidlaw QC, Brooks said the fact that private investigator Glenn Mulcaire - who has pleaded guilty to phone hacking - had been tasked with hacking Milly's phone was never brought to her attention before July 4, 2011.

She said she did not remember any discussion about Milly's disappearance when she checked in with her staff, and did not recall leaving a conversation in Dubai to take a phone call about the missing schoolgirl.

Brooks said the high level of contact between her and the NotW - including calls to the editor's office and texts with Coulson - while she was away was not "at all unusual", as she checked on the progress of that week's edition.

But she denied playing any part in the apparent removal of the text of a hacked voicemail message mistakenly left on Milly's phone by a recruitment agency, which appeared in the NotW's first and second edition on April 14, but later disappeared from the third, as well as a decision to move the article from page nine to p30.

Asked by Mr Laidlaw what she would have done if someone had told her on the Thursday or Friday that they had a lead on the whereabouts of Milly, Brooks said she would have told them: "Tell the police.

"I would have assumed probably that they would have told the police before me. If they had not, I would have told them to do so.

"If they had a lead that she was alive or they had found her, that would have been the correct thing to do."

Brooks told the court that she had never been asked to sanction the use of phone hacking to get stories while she was editor between 2000 and 2003.

But she admitted she did not actually know the practice was illegal, saying: "If you took my editorship of the News of the World at the time, I don't think anybody, me included, knew it was illegal.

"No one, no desk head, no journalist, ever came to me and said 'We're working on so-and-so a story but we need to access their voicemail' or asked my sanction to do it. It just didn't happen in the course of my editorship."

She said even though she did not know it was illegal, she would still have felt it was a " serious breach of privacy."

But she said although she had never sanctioned hacking, she might have done in the right circumstance and if there was a strong public interest.

She said: "If somebody had come to me with the right set of circumstances and asked me ... something to do with paedophiles, Roy Whiting... something along those lines... and had asked me with a good set of reasons, I may have done."

Earlier Brooks said she was unaware of a £92,000 a year contract the tabloid had with Mulcaire and said although the use of private detectives by Fleet Street at the time was "pretty normal", she had never heard his name before he was arrested.